Class &#34;b&#34; audio amplifier



6C W, l935 L. E. BARTON CLASS "B" AUDIO AMPLIFIER Filed Deo. 29, 1935PLATE SUP/:1. Y

IZUU- /fvl/E/vTo/a: Ln Earlow Cil Patented Dec. 10, 1935 UitiiTED STATESPATENT OFFICE CLASS B AUDIO AMPLIFIER Loy E. Barton, Collingswood, N.J., assignor to Radio Corporation of America, a corporation of DelawareThe present invention relates to electric discharge amplifiers and moreparticularly to audiofrequency power or output amplifiers of the class Btype.

In an audio-frequency power amplifier of the above type, a pair ofelectric discharge devices or tubes are disposed in balanced orpush-pull relation and are operated at substantially anode currentcut-off. There are, however, further distinguishing features of anamplifier of this type.

More particularly an amplier of the above type may be defined as amodified class B amplifier, or as a class B audio-frequency amplifier,as distinguished from a normal single ended class B amplifier employedin a tuned plate circuit radio-frequency amplier system, and having apower output proportional to the square of the excitation or input gridvoltage. The two types of amplifiers are similar only in that bothemploy tubes biased substantially to anode current cut-,olf and that theanode current increases in response to applied signal voltages.

The modied class B amplifier or class B audio-frequency amplier isfurther characterized by the fact that the tubes are arranged inpush-pull or balanced relation, so that each functions duringapproximately one half of each signal voltage wave or cycle. The tubesare further provided with balanced input and output circuits each havingan operating impedance substantially lower than the minimum input andoutput operating impedances of the tubes, whereby the tubes may bedriven by the signal potentiais far into the positive grid bias range ofoperation and may draw increasing grid and plate current to the limit ofthe space charge or emission for maximum power output.

A varying load is therefore placed upon the driving ampliiier or stagefor a class B audiofrequency amplifier and both the grid current and theplate current vary through wide ranges. High transient voltages may alsobe caused by steep signal wave fronts occurring in thev plate circuit ofa class B audio-frequency amplifier, because the normally low impedanceload above mentioned may become ineffective in response to the highfrequency transient signal waves.

It has been found that the leakage reactance of the output transformerfor the class B power amplifier in combination with the effective outputcircuit capacity across the primary or plate windings is responsive tothe high frequency oscillations set up by the transient signal voltagesand may often cause the output transformer of vention to provide animproved class B audiofrequency amplifier which is not subject to arcover or break down in the output transformer in response to signal wavescausing high transient voltages, and which is capable of handling agreater power output without distortion of the signal wave.

It is a further object of the present invention to provide an improvedclass B audio-frequency amplifier having an improved operating char- 15acteristic in response to signals having a high amplitude and arelatively wide audio-frequency range.

It is a still further object of the present invention to provide abalanced electric discharge ,0 amplifier of the class B" audio-frequencytype, having a balanced output circuit, for suppressing high frequencyoscillations which normally tend to produce break down voltages in theoutput transformer.

It is also a still further object of the invention to provide animproved balanced output circuit for a class B audio-frequencyamplifier.

The invention and the above and further objects thereof will be betterunderstood from the following description, when taken in connection withthe accompanying drawing, and the scope of the invention will be pointedout in the appended claims. l

In the drawing, Fig. l is a schematic circuit diagram of a class Baudio-frequency power or output amplifier embodying the invention, andFig. 2 is a curve diagram and amplier circuit illustrating certainoperating characteristics of the amplifier circuit shown.

Referring to Fig. 1, 5 and 6 are electric discharge amplifier devices ortubes suitable for class B audio-frequency amplifiers. In the presentexample, each tube is provided with a cathode l, an anode 8, and twospaced grid electrodes 9 and lil located between the cathode and theanode. These tubes are of the type known commercially as the RCA-46Radiotron.

The grid electrodes 9 and I0 are connected in parallel to form, ineffect, one grid or control 50 electrode in each tube. rIhe parallelgrid electrodes are then connected to the `opposite ends of a balancedinput circuit Il, provided in connection with a center tapped secondaryI 2 of a balanced signal input transformer i3. The

primary I4 of the input transformer is connected with suitable signalinput terminals indicated at I5 which terminals may be connected withany suitable source of ,audio-frequency signals (not shown) capable ofdriving the grids S-I to the limits of the positive operating range.

Under such conditions of operation, the grids may draw a relatively highcurrent and, accordingly, the input circuit must be capable of supplyingaudio-frequency power without signal wave distortions. For this reason,the impedance of the secondary I 2 is relatively low with respect to theimpedance between the grids and cathodes of each tube when driven to thelimit of the positive grid bias range.

It will be noted that the center tapped secondary I2 is provided with areturn lead i6 directly to the suitably center tapped cathode. A pair ofterminals I'I in the lead I8 are provided for introducing a biasingpotential if desired. With the present arrangement, the grids 9 mayreceive the proper biasing Apotential for essentially zero plate currentwith no signal. VIn this case, a double grid control is used whereby twospaced grid electrodes are connected in parallel, as one, between thecathode and the anode of each tube, and the normal anode current isreduced substantially to zero, without signal, for zero bias.

A class B audio-frequency amplifier of the biased type is shown anddescribed in my copending application, Serial No. 586,874,1iled January15, 1932, and a class B audio-frequency amplier of the zero biased typeis shown and described in my copending application, Serial No. 589,847,led January 30, 1932, both applications being assigned to the sameassignee as this application.

The electric discharge amplifier devices 5 and 6 are further providedwith a balanced output .circuit I8 and an output transformer rI3 havinga balanced primary winding 2D, provided with a center tap connection 2Iwith a supply lead 22 from a suitable well regulatedV source of platepotential indicated at 23. The negative end of the plate supply means isconnected through a cathode return lead 24 to the lead I8, whereby theplatepotential is applied between the cathodes and the anodes of thedevices 5 and 6 through the balanced primary winding 2li and the outputcircuit I8 connected with the outer terminals of the balanced primarywinding 28.

The output transformer I9 has a ratio such that the load, indicated at26 in connection with the secondary 21, reflected over into the primaryside of the transformer or into the anode output circuit, is of arelatively low impedance as compared with the lowest operating outputimpedance of either of the devices 5 and 6, whereby wide changes in theanode output current in response tosignals, as in the case of the gridsbeing driven to a maximum positive limit, may not cause appreciablesignal potential drop and distoition in the output circuit.

The operation of the circuits shown is such that each amplifier deviceoperates alternately on the positive half of each signal wave. Thenegative half of each signal wave is not effective to operate theamplifier, since the devices 5 and 6 are normally substantially at anodecurrent the curves 30 and 3l are grid current curves for normal andtransient voltage operation respectively of the amplifier in Fig. l,provided with bias potential at the terminals I'I. Each of the curves isplotted with respect to grid voltage along the horizontal axis andcurrent along the vertical axis.

These curves are plotted for electric discharge amplifier devices knowncommercially as RCA Ulf-849 for a high power audio-frequency ampli- Iierand with grid bias.

' It will be noted that the anode current may rise from substantiallyzero to over 1100 milli-amperes while the grid current varies from Zeroto over milli-amperes-indicating the wide range of variation of currentin both the input and output circuits of the class B amplifier.

In response to audio-frequency signals, having wide frequency range andhigh amplitude, abrupt changes may occur which cause high transientvoltages to be developed in the output circuit, sufficient to burn outorarc over the output transformer I9 as will be seen from an inspection ofthe curves given in Fig. 2, and for the reason that the outputtransformer has an appreciable leakage reactance which tunes witheffective circuit capacity across the primary 2), to respond to orresonate at relatively high oscillation frequencies. High transientvoltages having steep wave fronts are prevented from being transmittedfrom the load circuit by the tuned circuit comprising the transformerleakage reactance and the eifective circuit capacity in series, asindicated diagrammatically in dotted lines on the output circuit I8 at32 and 33 respectively in Fig. l.

The effective capacity across the primary winding in the anode circuitis constituted principally by the distributed capacity of thetransformer winding and the external circuit capacity which is chieflythe plate or anode to ground capacity, the effective value of which maybe higher than the static capacity when the circuit is in operation.

It has been found that the oscillations at the resonant frequency of theleakage reactance and its combining circuit capacity may effectively bereduced by damping and by-pass means in the output circuit, whereby thetransient voltages are reduced to safe values to prevent transformerbreak-down and to reduce distortion of the output wave.

The distortion may vary in different amplifiers depending upon the typeof amplifier devices used and the power to be transmitted thereby. Theeffect of such distortion will be noted particularly in the shape of thecurves 28 and 29 at the upper end as indicated at 33 and on the curve 3|at 35.

It has been found that the oscillations at the resonant frequency of theleakage reactance of the output transformer and the effective circuitcapacity in series therewith, may be damped to effectively reduce thetransient voltages by means of an impedance located adjacent to eachanode in the balanced output circuit, as indicated at 3S, and arelatively small capacity 3l between each anode andthe center tap 2I ofthe output transformer, thereby to make the impedance 36 effective ineach half of the output circuit at certain lower frequencies. Thiscapacity is effectively in series with the leakage reactance and circuitcapacity and in parallel across the output device between the anode andthe cathode. A closed circuit is thus formed with each half of theoutput circuit comprising an impedance, the output circuit capacity andan additional capacity all in than to 10% of the' load impedance in eachhalf i of the output circuit.

The capacity 3l effective in parallel with each amplier device should besuiciently large to permit appreciable currents at the lowestoscillation frequency to be transmitted through it, thereby to cause theimpedance or resistance in series with the anode to be elfective indamping the oscillations at the resonant frequency of the leakagereactance and the effective circuit capacity.

It will be noted that the condensers 31 are connected between points inthe output circuit across which exists a relatively low direct currentpotential diiference. The connection, as shown, is between each anodeand the center tap of the output transformer, whereby the potentialexisting across the condenser in each half of the output circuit is onlythat resulting from the potential drop through each half ofthetransformer primary winding and the still lower potential dropthrough each of the impedances 30.

It will be seen therefore that the condensers 3l may be of relativelylow cost for the reason that ordinarily they need be insulated only forlow direct current potentials, as compared with potentials of the platesupply, which may be in the order cf several hundred volts, although thecondenser is effectively connected between the anode and the cathode ofeach amplier'device. However, the alternating current potential or audiofrequency voltage across the condensers may be high. In the presentexample, the condensers have the full alternating current or audiofrequency voltage which is 2500 volts in this case and when thecondensers are effective in suppressing oscillations they must passappreciable current at frequencies above the audio frequency band.However, in general, the condensers are of relatively low cost becauseof the small capacity required.

It will be seen that each half of the output circuit is in itself, aclosed oscillatory circuit provided with individual damping means ofrelatively low impedance. In a class B audio-frequency amplifier, theentire output circuit must be of relatively low impedance compared withthat of the amplifier devices and it will further be seen that ierelative impedances of the devices 35 and the output circuit are suchthat the additional resistance in circuit may not appreciably aifect theshape ofthe signal wave or the power output of the amplifier. In theexample given, the additional '75 ohms in circuit with 2500 ohms doesnot materially increase the voltage distortion in the circuit.Furthermore, because of its uniform frequency characteristic,non-inductive resistors are preferable as the impedance devices.

By utilizing a circuit in accordance with the invention, all of theadvantages of the efficient maximum power handling ability andsubstantially distortionless operation of a class B It is commonknowledge that Iin `general an.

audio-frequency transformer such as the transfermer I9, designed totransmit audio-frequency signals from 30 to 15,000 cycles hascomparatively high leakage impedance to signals in the frequency rangeof 20,000 cycles and higher. The fact that leakage impedance is presentdue to leakage inductance and is relatively large at high frequenciesmakes it obvious that the primary circuit cannot effectively be loadedby the secondary. Therefore, the primary load becomes essentially aninductance 32 and capacity 33 in series'and have a resonant period whichin general occurs at frequencies above the audio frequency range. Itwill be seen that since little effective resistance is present toprevent high ,currents through 32 and 33 the voltage across theseimpedances may become relatively high if the circuit is excited at itsresonant frequency. The sharp bend of the curves 29 and 28 at the upperend due to over excitation of the grid and the abrupt rise of the gridcurrent at about the same time introduce harmonics in the plate circuit.The negative bend of the grid current curves also cause .abrupt changesin the plate current curve. These abrupt changes for low frequencies,say 200 cycles, may result in harmonics well within the audio range butat higher frequencies, for example 5000 cycles, or even down to lowfrequencies, if the plate current changes are sumciently abrupt theharmonics introduced may easily fall within the resonant frequency rangeof the above mentioned impedances, in which case high voltage transientsappear and with the transients or damped oscillations increaseddistortion results'.

Since the high voltage actually appears across the terminals of thetransformer primary, a capacity across the primary rsuch as theeffective plate capacity and additional capacity 3l changes the resonantfrequency but has little effect upon its amplitude unless the frequencyis changed into the region in which the secondary load would beeffective on the primary. However, if a resistance of comparatively lowvalue is added as at 353 the loading at the resonant frequency of thesystem easily suppresses the high frequency resonant voltages.

I claim as my invention:

` 1. In a class B audio-frequency amplifier, the combination of anoutput transformer having a primary winding provided with a center tapconnection, a pair of electric discharge amplier devices having an anodeoutput circuit connected in balanced relation to said primary winding,said transformer providing an output load impedance lower than thenormal output operating impedance of said devices and having leakagereactance and distributed capacity responsive to high frequencyoscillations, means providing a series irnpedance of relatively lowohmic resistance in each half of said balanced output circuit and meansproviding a shunt capacity across each half of said output circuit inseries with said leakage reactance.

2. In a class B audio-frequency amplifier, the combination of an outputtransformer having a primary winding provided with a center tapconnection, a pair of electric discharge` amplifier devices having ananode output circuit connected in balanced relation to said primaryWinding, said transformer providing an output load impedance lower thanthe normal output operating impedance of said devices and having leakagereactanceV and distributed capacity responsive to high frequencyoscillations, and means for damping said high frequency oscillationscomprising a series impedance device of relatively low ohmic resistancein said output circuit adjacent rto each anode, and a by-pass capacitorconnected between the anode end of eachy of said impedance devices andthe center tap of said primary winding.

3. In a class B audio-frequency amplifier, the

combinationwith a pair of balanced electric discharge amplifier devices,of a balanced output circuit therefor comprising an output transformerhaving a center tapped. primary winding and Ahav- Y ing a predeterminedleakage reactance and distributed capacitance, an impedance device of10W ohmic resistance connected in said output circuit adjacent to eachof said amplifier devices, and a by-pass condenser connected between thecenter tap of the transformer winding and each of said impedance devicesAto complete a damped oscillatory circuit through each of said devicesand one half of said transformer winding.

4. In a class B audio-frequency amplifier, the combination with anoutput anode circuit therefor and an output transformer in said circuit,of means for damping, high frequency oscillations in said circuitcomprising a resistor in each half of the balancedl output circuithaving a resistance value in ohms substantially lower than the ohmicimpedance of the output circuit, and means providing an eliective anodeto cathode capacity across said output circuit to resonate the leakagereactance in said circuit at a frequency above the effective audiofrequency range of said amplifier.

5. In an audio-frequency amplifier, the combination with a pair ofelectric discharge amplifier devices having a balanced output anodecircuit, of an output transformer in said circuit providing a loadtherein of relatively low impedance with respect to the output impedanceof said devices, and an impedance device in each half of said balancedoutput circuit adjacent to the output anode of each of said devices,having an ohmic resistance less than 10% of the impedance of said outputcircuit in series therewith.

6. In an audio-frequency power amplifier having low impedance balancedinput and output circuits for a pair of electric dischargevpoweramplifier devices, of means for damping oscillations at the resonantfrequency of the leakage reactance and eiiective capacitance in theoutput circuit thereby to reduce transient voltages across said outputcircuit to desired ope-rating values, said means comprising an impedancedevice in said output circuit adjacent to each of said amplifier devicesof an impedance value less than 10% of the load impedanceof said outputcircuit.

'7. In an audio-frequency power amplifier having low impedance balancedinput and output circuits for a pair of electric discharge power amplierdevices, of means for damping oscillations at the resonant frequency ofthe leakage reactance and effective capacitance in the output eachhaving an anode, a cathode, and a control grid, said devices beingconnected in balanced relation to each other and being adapted to bebiased substantially to anode current cut off, an input circuit of lowimpedance with respect to the grid impedance of each of said deviceswhen g drawing a predetermined maximum grid current, an output circuitof low impedance with respect to the anode impedance of each of saiddevices, and a resistor connected electrically directly with each anodein series with the output circuit, said i resistors each providing animpedance substantially lower than that of said output circuit.

9. An audio-frequency amplifier including, in combination, a pair ofelectric discharge devices each having an anode, a cathode, and acontrol grid, said devices being connected in balanced relation to eachother and being adapted to be biased substantially to anode current cutoff, an input circuit of low impedance with-respect to the gridimpedance of each of said devices when drawing a predetermined maximumgrid current, an output circuit of low impedance with respect to theanode impedance of each of said devices, and a resistor connectedelectrically directly with each anode in the output circuit, saidresistors each havingY an impedance substantially lower than that ofsaid output circuit and means connected to said output circuit foreffectively increasing the anode to cathode capacity of said circuit.

10. A class B audio-frequency amplifier including a pair of balancedelectric discharge devices each having a cathode, an anode, and acontrol grid structure connected with the cathode and so spaced betweenthe cathode and anode thereof that said devices operate at substantiallyzero normal anode current and bias potential, a balanced output circuitconnected between said anodes and including a balanced outputtransformer having a center tapped primary winding in said circuit, aresistor connected between each I anode and said primary winding anda'by-pass condenser connected between each anode and the center tap ofsaid primary Winding, said resistor beingvle'ss` than 10% of theimpedance of the output circuit. 11. In an audio-frequency ampliiier,the combination with a pair of electric discharge ampli- Iier deviceshaving a balanced output anode circuit, of an output transformer in saidcircuit 6 providing a loadtherein of relatively low impedance withrespect to the output impedance of said devices, and said transformerhaving a leal:- age reactance and an effective capacity across theprimary winding-thereof whereby it is responsive l to high frequencyoscillations set up by transient signal voltages and an impedance devicein each half of said balance output circuit adjacent to the output anodeof each of said devices having aV resistance value in ohms substantiallylower than the ohmic impedance of the output circuit.

12. In a class B audio-frequency amplifier, the combination with anoutput anode circuit therefor and an output transformer in said circuit,of means for ydamping high frequency oscillations in said circuitcomprising a resistor in each half of the balanced output circuit havinga resistance value in ohms substantially lower than the ohmic impedanceof the output circuit, and a capacity connected between each anode endof said output anode circuit and a center tap on the primary of saidoutput transformer.

LOY E. BARTON.

